One Princess And A Coupla Queens

Which one is grammatically correct or better? I have two assignments, one of them is done. I have two assignments, one of which is done.

Does but one mean only one or except one? This phrase shows up in the song love is an open door from the movie frozen. The relevant line is our mental synchronization can. The one could imply that of the alternates only one choice is possible, or permitted. Which alone could indicate several choices from the set of alterates could be selected in various.